The present invention relates to a press assembly and method of operating the press assembly. Specifically, the invention relates to operation of a cushion assembly and/or an auxiliary apparatus in a press assembly.
There are many known press assemblies which are used to deform workpieces. These known press assemblies may have an upper die which cooperates with a lower die to deform the workpiece. Workpiece holders may be associated with the upper and/or lower dies. Press assemblies having known constructions are disclosed in U.S. Pat. Nos. 1,884,700; 2,217,172; 3,296,850; 3,456,478; 3,636,748; and 3,636,749.
Stretch drawing is performed during the operation of some known press assemblies. During stretch drawing, the edges of a sheet metal workpiece are firmly gripped between upper and lower draw rings. After the workpiece has been gripped, an upper die is closed against a lower die to deform the workpiece. Under certain circumstances, a stretch draw operation is preferred because it reduces the total amount of metal used to form an article, tends to improve the quality of the article, and facilitates maintaining uniform quality during production of a series of articles.
In an effort to avoid noise, vibration and shock loading of components of a press assembly during operation of the press assembly from an open condition to a closed condition, U.S. Pat. Nos. 4,499,750 and 4,635,466 suggest that a lower draw ring or sheet metal holder be accelerated in a downward direction before an upper draw ring comes into contact with the workpiece. In the case of U.S. Pat. No. 4,499,750, downward acceleration of the lower draw ring is effected by actuation of a control valve in timed relationship with opening of the press. Actuation of the control valve exposes a piston to fluid pressure to move the lower draw ring downwardly against the influence of a die cushion piston.
In the case of U.S. Pat. No. 4,635,466, a pilot pin is engaged by an upper die and moved downwardly to compress hydraulic fluid in a chamber connected with a lower bolster. The fluid is exhausted from the chamber through a choker restriction so that as the pilot pin move downwardly, the fluid pressure in the chamber causes the lower bolster to be moved downwardly.
During operation of a press assembly from a closed condition to an open condition, upward movement of a lower die has been retarded to prevent the lower die from moving upwardly simultaneously with upward movement of the upper die and a striking of the bottom of a formed workpiece on the upper die before the workpiece has been released. In U.S. Pat. No. 3,456,478, fluid flow from a locking cylinder chamber is blocked to delay upward movement of a lower die until a predetermined point in an opening cycle of the press assembly. By delaying upward movement of the lower die, striking of the formed workpiece before the blank holder is released is prevented.
During operation of a press having a draw ring, it has been suggested that return movement of the draw ring be retarded with respect to the return of an upper die of the press. In order to retard upward movement of the draw ring, it is suggested in U.S. Pat. No. 1,638,881 that fluid pressure be applied against a relatively small surface area on the upper side of a piston connected with the draw ring. The fluid pressure on the lower side of the piston is able to move the draw ring upwardly with a reduced force which is just sufficient to move the workpiece or shaped article from a lower die. The operation of a valve for controlling the application of fluid pressure to the upper side of the piston is controlled as a function of movement of the upper die of the press assembly.